The surface hardness and compressive strength of dental stone and improved dental stone were investigated when crystallised in contact with impression materials. Vickers hardness tests were performed at 45 min, 2 h and 24 h, and compressive strength tests were performed at 24 h after the beginning of stone mixing. For the surface hardness of stone models, the impression material type, stone type and measurement time were found statistically significant (p<0.001) and for the compressive strength, the impression material type and stone type were found statistically significant (p<0.001). Within the measurement times, the hardness values were statistically different from each other. The highest hardness values were obtained from the 24 h measurements, followed by the 2 h measurements, while the lowest hardness values were obtained from the 45 min measurements. Specimens obtained from improved dental stone showed higher hardness values and compressive strength than those from dental stone.